"Monogamous couples" refers to couples who are committed to each other exclusively, meaning they choose to be in a relationship with only one person at a time. Full definition
While most monogamous couples agree that having a sexual relationship with a person outside the relationship constitutes cheating (not all - more on polyamory and open relationships here), there is a growing understanding that it is possible to have an emotional affair, even without sexual contact. (nancybrittaincounseling.com)
For monogamous couples who have agreed to only have sex with one another, finding out that a partner has either sought a sexual affair or has actually followed through with an affair almost certainly shatters all trust within a relationship. (counselingatl.com)
An open relationship certainly seems alluring on the surface: sexual gratification and novelty without the harm or shame of betrayal, especially for long - term monogamous couples who are no longer sexually satisfied. (mindbodygreen.com)